Arc welding is a process of joining metals by applying an arc to provide filler material in a molten metal pool or puddle on a workpiece. Various arc welding methodologies have been developed in which material from a consumable welding wire or electrode is melted and transferred to the workpiece. Many arc welding processes, such as metal inert gas (MIG) techniques employ a shielding gas around the welding arc to inhibit oxidation or nitridation of the molten metal. Non-inert shielding gases such as CO2 may also be used, whereby such processes are sometimes generally referred to as gas metal arc welding (GMAW). Other arc shielding processes similarly provide a protective shield of vapor or slag to cover the arc and molten weld pool. In the case of MIG welding, the molten material may be transferred from a consumable welding wire or electrode to the workpiece by several mechanisms or processes, including short-circuit welding, spray arc welding, and pulse welding. Short circuit welding techniques involve electrical connection of molten metal to both the electrode and the weld pool during a portion of each welding cycle, wherein the molten material contacts (is electrically shorted to) the workpiece or the weld pool thereof prior to separating from the electrode. This type of welding is prone to spatter that disrupts the weld pool and/or to cold lapping where there is not enough energy in the puddle for the filler material to fuse properly to the workpiece. In addition, short-circuit welding techniques suffer from low deposition rates compared with pulse or spray welding. Non-contact or non-short circuit welding approaches involve transfer of molten metal from the end of the electrode across the welding arc to the workpiece through electromagnetic forces, wherein the electrode ideally never electrically contacts the workpiece (no short-circuit condition). Non-short circuit welding includes so-called spray arc and pulse welding processes. Spray arc welding is a relatively high energy process in which small molten droplets are propelled from the electrode to the workpiece, typically employing a constant voltage (CV) to produce enough current to send a constant stream of metal off the electrode at a rate of hundreds of droplets per second. This technique exhibits rather high heat input and is useful only over a limited range of welding positions. Spray welding is also prone to burnthrough on thin workpiece materials.
Pulse welding offers an alternative non-contact process for electric arc welding that utilizes lower heat to generate a less fluid molten metal puddle on the workpiece. This facilitates out of position welding and improves various mechanical aspects of the welding process, without the high spatter issues of short-circuit welding and without the risk of burnthrough found in spray welding, particularly for thin workpieces. Pulse welding is performed by high-speed manipulation of the electrical signal applied to the electrode and is designed to be a spatterless process that will run at a lower heat input than spray or globular transfer methods. In general, pulsed MIG processes involve forming one droplet of molten metal at the end of the electrode (a melting condition) and then transferring the molten material using an electrical transfer pulse (a transfer condition) in each of a sequence of welding cycles, where the droplet transfer occurs through the arc, one droplet per pulse, without short-circuiting the electrode to the workpiece. Unlike constant voltage welding processes, pulse welding employs a high energy pulse to initiate the transfer condition in each welding cycle, and the welding current is then dropped to a background current level to begin melting the end of the electrode to form the next molten metal ball. In this regard, pulse welding allows the workpiece to cool after each molten ball is transferred to the weld pool, whereby pulse welding is less susceptible to burnthrough for thin materials than is spray welding. Moreover, pulse welding does not suffer from spatter problems or cold lapping, as is the case for short-circuit welding. As the electrode advances, the pulse welding process transfers small droplets directly through the welding arc, with the objective being one droplet during each pulse.
Ideally, a molten metal droplet or ball is formed on the end of the electrode by electrode heating from the background current, and is thereafter transferred across the arc to the workpiece by the high current pulse without short-circuiting. The pulse preferably causes the molten metal to separate from the electrode by an electric pinch action, after which the molten metal mass or droplet is propelled across the arc to the weld pool of the workpiece. In this regard, the energy in the current pulse used for separating and propelling the molten metal to the workpiece is an important parameter of the overall pulse welding process. The electric pinch action exerted on the droplet to constrict and separate the droplet from the electrode is roughly proportional to the square of the applied current during the current pulse, and to a point, higher pulse current during droplet separation results in more rapid transfer to the workpiece and consequently a superior welding process. However, the arc current also exerts a magnetic force on the molten weld pool on the workpiece, pushing the weld puddle downwardly away from the end of the electrode, wherein this downward force may push the molten metal outwardly and cause a puddle depression below the electrode. This depression and the associated electromagnetic forces can cause extreme weld puddle agitation for high pulse current levels, especially when welding metals aluminum or other material having low specific gravity, leading to a poor weld bead appearance and excessive penetration of the metal into the workpiece.
Thus, the ball separation pulse needs to be tailored to accurately control the pinch action, while minimizing the puddle agitation, wherein the magnitude and shape of the current pulse is ideally set to provide a smooth metal transfer with a minimum puddle agitation. This, of course, is a tradeoff, wherein a pulse that does not contain sufficient energy may lead to short circuit conditions and the associated spatter problems. In particular, a relatively weak pulse may fail to fully separate the molten metal from the remainder of the electrode before the ball engages the weld puddle, causing a substantial amount of spatter. Thus, the electric pulse must have a certain minimal amount of energy to allow efficient transfer of a given amount of molten metal to avoid short circuit conditions. However, if the energy in the current pulse is too great, severe puddle agitation occurs. Because of this inherent tradeoff, the length of the current pulse is commonly extended in order to ensure ball transfer without short circuiting, while permitting some amount of puddle agitation and/or extra workpiece heating. Such overcompensation to avoid short-circuit conditions, however, is not universally acceptable, particularly for more susceptible processes, such as very thin workpieces. Furthermore, the welding pulse parameters may need to be tailored to produce a stable arc with a minimum arc length and spatter, for a given wire size, chemistry, blend of shielding gas, and wire feed speed. Non-optimal electrode current waveforms results in excessive spatter or an excessive arc length, wherein a long arc can lead to a contaminated weld and reduced overall welding travel speed.
In addition to crafting the pulse amplitude and duration, the background current and the duration of the melting condition may need to be adjusted for an effective pulse welding process. For instance, the background current level generally affects the overall heat provided to the workpiece and also controls the molten ball formation on the electrode tip. In addition, some of the pulse energy may also operate to melt electrode material prior to ball separation. The electrode heating includes resistance heating by current flow through the wire from the wire feeder electrical connection (holder) to the end of the wire, as well as anode heating at the end of the wire, which varies with the effective arc current, wherein the anode heating generally contributes the majority of the melting energy during each welding cycle. In this regard, as the extension or stick-out length (e.g., the distance from the holder to the end of the electrode) increases, a larger portion of the heating per cycle is resistance heating caused by current flow through the welding wire. Conversely, as the stick-out decreases, less heating is by resistance heating of the wire. With respect to variations in the size of the transferred material, if the molten ball is too small at the beginning of the transfer condition, the pulse current may cause the ball to be “stretched” or pulled as pinch forces attempt to detach the droplet, in which case the bottom of the molten ball can contact the weld puddle (short-circuit), resulting in spatter. Conversely, if the formed molten ball is larger, the pulse current will tend to detach the droplet without “stretching” the molten mass. Thus, for a given welding process, the pulse shape, and the level and duration of the background melting current are preferably selected or adjusted such that only a minimum amount of the pulse energy contributes to additional electrode heating, wherein the pulse current wave shape essentially serves only as a means to detach the droplet. However, this situation is only achievable if the volume of the molten metal ball is repeatable and uniform for each welding cycle at the time the high current pulse is applied. In conventional pulse welding processes, the welding waveform (e.g., background current and high current transfer pulse) is repeated in a series of welding cycles without variation, wherein the fixed waveform is preferably selected to achieve the proper ball size, arc length, and transfer characteristics in each cycle to provide good performance and weld quality. However, process variation is inevitable as conditions, materials, temperatures, etc. change over time or from one workpiece to the next. Consequently, there is a need for improved pulse welding methods and systems by which repeatable high speed and high deposition rate pulse welding operations can be achieved for a given transfer pulse and background current welding waveform without short-circuit conditions and without weld pool contamination or puddle agitation.